Dengue virus (DENV), which exists as four closely related serotypes, is a single-stranded RNA virus in the flavivirus genus. With the global resurgence of DENV infections, including the DENV-1 outbreak in Key West, Fla. (CDC, MMWR 59, 577-581, 2010), dengue has evolved into one of the world's most important arboviral diseases. DENV infection causes either mild dengue fever, or severe life-threatening dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS). Severe dengue is a common occurrence in children residing in hyperendemic countries and is strongly associated with secondary heterotypic infections (Sangkawibha et al., Am J Epidemiol 120(5): 653-669, 1984). Currently, vector control and education programs are all that are available for dengue disease prevention; and the development of dengue vaccination has been hindered by concerns of waning or imbalanced tetravalent immunity leading to vaccine induced DHF/DSS. However, a handful of vaccines are in the early stages of clinical trials (Durbin and Whitehead, Curr Top Microbiol Immunol 338: 129-143, 2010).
DNA vaccination has become a fast growing field in vaccine technology since the 1990s following the first reports of plasmid DNA inducing an immune response to plasmid-encoded antigen (Tang et al., Nature 356(6365): 152-154, 1992). Although DNA vaccines are considered by some to be one of the most important discoveries in the field of vaccinology (Mor, Biochem Pharmacol 55(8): 1151-1153, 1998), DNA vaccination in most cases is hampered by low immunogenicity and efficacy. Thus various strategies to improve the immune response following DNA vaccination have been developed. Earliest attempts to increase DNA vaccine immunogenicity have included optimization of route, dosage, and timing of administration; DNA encoded or exogenously administered costimulatory molecules and cytokines; and prime-boost regimens (Leitner et al., Vaccine 18(9-10): 765-777, 1999).